PCPartner VTech VP 486 SX 25MHz Mainboard 4MB 30-Pin, AT PSU, 128KB Cache DOS

This motherboard has an integrated 486 25MHz CPU, but there is a socket where you can install a DX CPU if desired. 128KB cache, tested. The switch I used was smaller - so I covered the little gap by gluing a small bit of plastic which also sits under the switch to keep it secure.

eBay