Obviously levelling the slab to begin with would help here, checking in both directions and along both diagonals with a level1.
but this time I think a little silicone/hot glue boundary might keep the epoxy from running over again
You want to be very careful using anything containing silicone around woodworking. Silicone contamination is notoriously difficult to remove (and to contain — it can spread from piece to piece with handling) and as a result it's not uncommon for people to avoid using all products that contain it anywhere in the shop.
Hot glue would certainly work, but it's going to require quite a large quantity so not sure if that's a cost-effective solution, particularly if buying the gun especially for this. And there are cheaper options anyway.
Ultimately, my requirements are simple:
- Does not bind to the epoxy
- Can be removed easily
- Won't stain wood (I will be planing afterward, so this doesn't matter as much).
Tape. Tape is your friend when working to epoxy in multiple ways, including protecting adjacent surfaces and making up dams for the back of holes that go right through the wood and for when a void runs out the side or end of a board2.
Epoxy does bond slightly to the face or edge of some tapes, but IME it's never so bad that it's a big deal and the normal cleanup that you're going to do anyway to make the epoxy flush with the wood will take care of any issues. Epoxy can also creep under the edge of some tapes if not burnished down well but it's usually minor.
Basic masking tape works all right with epoxy, although I believe that painter's tape works slightly better. But duct tape works best of all (including cheap non-brand stuff) because epoxy won't bond to soft plastics, so this would be my recommendation. And duct tape has another advantage for you here in that it's the thickest of these three options.
Because tape is conformable to a degree you could couple it with thin wood strips (coffee stirrers would be great for this) to build up a dam of significant height really quickly rather than having to use multiple layers of tape.
Worth filing away in the mental bank for future projects, dams around epoxy pours can also be made using oil-based modelling clay (Plasticine equivalent), Play-Doh and Blu-Tack and the majority of the material can be recycled for future use.
Note that all of these can leave a slight residue on wood but again this would usually be taken care of by the standard post-pour cleanup to flush the epoxy.
1 Which these days might be your smartphone (yes, there is an app for that)! If using a phone it would be best to extend its baseline by using it on a longer piece of scrap wood or ply that you know is dead square and flat.
2 Somewhat paradoxically epoxy won't bond to the sticky side of tape.