I needed a bracket to hold a 7w 14cm x 28cm vivarium heater to the wall of my new Ikea samla dry box so threw this together.  The heater can be found here or on eBay: https://www.amazon.co.uk/gp/product/B0817VP88T/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&th=1  Two at either corner should be enough.  

The holes are 3mm and 7mm deep.  They're too big to take an M3 screw - I should have designed a hole on the other side to take a hex nut but that seemed like overkill.  However, with a bit of force you can screw a short M4 screw into it and it works fine.  Don't screw in the screws while the heating pad is in place as the thread might pierce the pad.

Best way to use it is to put the heater in place in your box, put one corner around it and mark where the screw holes are from the outside with a pen, then drill with a 3mm drill bit.  Once one corner is secure, move the heater pad into place and then put the opposite corner in place, mark, drill and screw as before.  

Before you can do any of this though… you'll need to take the heater pad rheostat apart, unsolder the neutral leads and thread the cable through a 5mm hole in the side of your box!  Once it's all in place, solder it back together and seal around the hole for an airtight box.  As I'd already done this once to the heating pad for a previous dry box, when I did it a second time the PCB tracks came away so I've had to bin the heater and order a new one.  Once I've installed it, I'll upload some pictures of it in place.  Meantime, the picture I've got is of the brackets in place on their own.  Once the heating pad is in place I just need to make a container for my silica gel pouches and my dry box is complete.

Designed at speed on Tinkercad.  Prints out at 15% infill; use PETG: it's holding a heater so PLA won't be good enough.  

UPDATE: I've uploaded a version that holds the heater a bit further from the side of the dry box as the first design seems to be heating the side of my dry box more than I'd like.  It will hold the heater 2cm away from the side now.