I get where Michael is coming from but i would be tempted to keep things simple (unless its being re-mounted over and over)

you could see how you get on with it, that way you may be able to order the sizes you need with minimal hassle on your part to get them machined

going so deep with those bolts wont add much if anything to the plates rigidity, id say 40mm would give as much as 80mm.

an advantage with forgoing the slot is you will have a touch of play to correct any tiny errors that you may have built in elsewhere

a couple of simple Ali blocks could be added as buttresses if it turns out not to be rigid enough... it will look a tad utilitarian but I'm guessing the function supersedes the form and your struggling to get them machined :)
if your paying someone then of course get nice triangular buttresses :)

buttresses will ad an ENORMOUS amount of rigidity to the plate compared to just the bolts but with a 2kg load i would think the bolts would be fine, maybe add two shorter bolts between the outer two?