I recently got one of Paul Churnock's bed extension kits. It's a really nicely made kit and works well. The only issue I had was that my MP Mini Select had the new style bed rails that are tapped and directly mounted to the bed with M4 screws. Paul's kits are designed to utilize the original style bed rail mounting brackets that were supplied with the printer. I created these brackets so I could easily mount my new extended bed.

You will need:

8 - M3 x 10 bolts
4 - M3 x 8 bolts
12 - M3 nuts

The original MP rail mount brackets used an M# bolt that was tapped directly into the bottom of the mount. I preferred to design my version using a trapped nut that was accessible from the sides or the bed, which increases the clearance under the bed frame. The brackets are designed with slots on both sides of the rails. Just use the outer holes.

The brackets are designed with very tight tolerances. My first layers created an overly tight fit and needed a bit of cleanup. Your print setup might need minor adjustments.

Do not tighten everything down at once. Assemble the rails with the bolts just finger tight. then clamp the bed down to a know flat surface. I used my table saw bed with a pair of Quick-Clamps. Then tighten everything down. Otherwise you risk warping the bed, which would not be a good thing.

If you are using these as replacements for your original mounts, I would replace all 4 to keep distances and tolerances the same.