If you mean a "slot" for the left-side A500 edge connector to plug into, I've used an ISA slot for this before. You need to cut it where the spacer inthe ISA slot is to remove that, size the two ends to the proper number of contacts, and then I used liquid epoxy to glue it together and a large paperclib embedded into the epoxy on each side for strength. If you have a spare A500 or old edge connector of some sort thet's suitable size you can use that plugged into the two ends to hold things straight and lined up with edge connector metal while the epoxy cures.
I've built a few of these over the years, seems to work well. I've also made a couple Zorro-size slots this way, though you need two ISA slots for that one. Sometimes a bit of epoxy can get into the connector springs at the seam between the two ends, but if you're careful and keep an eye on those things and clean out any gunk before it's solid it should be fine.
