This is the launch tower I designed as a general purpose launcher for all of my high performance rocket designs. It is configurable for rockets between 1" and 5" in diameter, but is really intended for 29-98mm minimum diameter rockets. This tower has served me well on flights from H through high impulse Ms, and I have no doubt in my mind that it couldn't handle a 98mm O motor.

All hardware used in the tower design is 1/4-20. 

The tower uses either 1.25" OD Aluminum tubes, or 1-1/4" EMT conduit, which has an OD of 1.51". In order to interface with the tubes, I use star nuts, McMaster part number 94290a571 for the 1.25" Aluminum, and number 94290A591 for the 1.51" OD conduit.

The first major choice you need to make is what tubing to use. I originally used anodized 1.25" Aluminum tubes from my local metal supplier, and they worked pretty well. I did notice some damage to the ends of the tubes near the base of the tower after flying some big motors out of it, but nothing too concerning. The 1.51" OD EMT conduit tubes are more robust, but they are significantly heavier, and have a rougher surface finish. For length, I'd suggest at least 6 feet long, which was what I could get the Aluminum tubes in. For the EMT conduit version, I chose 8.5 feet long, as that was the longest that I could practically transport.

To start tower construction, start by getting the top ring, arms, and baseplate cut out. I personally chose to get the top ring and arms cut out of 0.25" steel, and the baseplate cut out of 0.5" steel. The holes in the top plate are intended to be drilled and tapped for 1/4-20 threads, if you order the plate from a company like SendCutSend, I'd strongly suggest having them do the tapping, it can be a bit of a pain point. Also, if you order from a service like SCS, try to group some orders with a friend. Getting multiple copies of a part made can lead to dramatic per part savings even if it's just going from one to two parts. If you get the parts cut out of regular steel, they should be painted to prevent rust. I haven't tried other materials, but even Aluminum will probably be sufficient for the top ring and arms.

First ensure that they are all straight, the same length, and that their ends are flat and perfectly perpendicular to the axis of the tubes. I like to use a tubing cutter to trim the ends of the conduit. Next you need to install the star nuts into the ends of the tubes. First print off the appropriate star nut installation tool. Then slide it over each end of each tube and make a mark where it stops. Then insert a star nut into the tool, and screw a bolt through the center hole into the star nut. When looking down the tool the nut should appear convex. Then simply slide the tool over the end of the tube, and hit the bolt head with a hammer until the tool reaches the mark you placed earlier. Then simply unscrew the bolt and repeat until all four tubes have a nut installed in each end.

 While they aren't strictly necessary, I strongly suggest printing out two of the appropriate tower spacers for your rocket size. There is an example spacer design in the files, and here is a parametric Onshape model, simply input the diameter of your tower tubes, and your chosen rocket diameter and it'll generate 3 fin and 4 fin tower spacers. The rocket diameter you input should be slightly larger than the actual diameter of your rocket. I generally go about 0.04" oversized, but it honestly depends on what works for you.

For tower assembly, a table slightly shorter than the length of the tower tubes helps a ton. First bolt one of the tower spacers to the bottom plate via the center hole. Then bolt the tower arms to the top plate, use the second spacer to get them set to approximately the correct position. Then start securing the tubes to the bottom plate and tower arms. I like to start by securing two adjacent tubes to the bottom plate, then bolting the top assembly to them, followed by installing the second pair of tubes. Honestly the whole assembly process can be a bit of a pain, but it works well enough. Finally, once all of the tubes are installed, loosen the bolts holding the arms to the top plate, slide the spacer in near the top and then hold the tubes tightly around the spacer and tighten the bolts going into the top plate, then remove the spacer.

For setting up the tower, I use three or four ratchet straps and stakes. It's quite helpful to have a second person holding the tower upright. If you choose to use tubes on the longer end, you will probably want to hook the straps to the top ring before lifting it vertical. I like to get it pretty vertical, stake down all the straps, tension the straps, and then adjust the angle of the tower by kicking around the bottom plate.