Thanks for all the advice.

One question about putting the bricks under the legs of the cot. Did you find it difficult to raise and lower the side rail? With our Boori cot you have to push the side rail inwards and then it releases to lower it. Just a little concerned that the cot might become unstable on the bricks when I push in IYKWIM?

Otherwise I think we'll try putting a pillow or something under the mattress.

I think I'll try some side positioners too as bub likes to sleep on his side when we co-sleep. Thanks for the ideas!
Mine requires me to press two buttons on the side, lift the rail a bit and then it drops and it is fine, we have a thick carpet (not a loop pile) that we wrap around the bricks so the legs settle into it really well. But test it out and if you aren't comfortable or it moves try one of the other suggestions We have a Mali Euro cot, solid timber and really heavy so it doesn't move