Presuming your RAID is Thunderbolt 1 or 2, technically the term for the adapter is a Thunderbolt 3 to Thunderbolt 2 adapter as USB-C is the connector type. You've probably looked around and have seen that non-Apple alternatives are actually more expensive. I haven't seen a cheaper adapter (unless there's a sale or you get it used), nor have I seen a Thunderbolt 3 to Thunderbolt 2 cable and Thunderbolt 3 hubs I've seen don't include Thunderbolt 2 ports. Like you, if somebody knows of a viable cheaper alternative, I'd like to know. You can't just wire up the connectors as Thunderbolt cables and adapters have electronics in them.
The cheapest alternative is probably to look for used or "refurbished" Apple adapters as a lot of people buy (or at least they used to, based on the customer reviews) the adapter thinking it will connect to a DisplayPort monitor and then return them or sell them when they find that that's not the case.