Is the relay removable or is it integrated in to the fuse box? If removable, you should be able to pretty easily source a relay that will either fit perfectly or can be cable tied somewhere and connected with jumper wires. If integrated you can either disassemble the panel and remove the old relay, the do the jumper wire trick and relocate the relay external to the box or possibly find the appropriate wires leading in to the panel and tap in to them externally.
-Rod