Another technique to consider, if you just need to extend MGL-supplied wires that are pre-terminated with D-Sub pins or sockets, is to crimp the mating pins or sockets to the extension wires, mate the terminals and cover with adhesive-lined heat shrink. Offset each connection by an inch or so to keep bundle diameter to a minimum.
Links for D-Sub pins and sockets are at item 3, in post #3, above. The links to eBay are dead; here are new ones to search for pins and sockets.