tldr; the SpiderVPN Classic is the ZBT-WR8305RT, and the SpiderVPN Pro is the ZBT-WE1326.
I found these routers on Amazon and Ebay, for next to no money.
The SpiderVPN Classic I picked up for £18 GBP, it has 4 antennas, 64mb ram, 8mb rom, 5 x 100mb ethernet ports, and was running a custom build of Openwrt built in April 2019.
You can upgrade from the web interface to Openwrt 18.06.2 which factory resets the device.
So as this was so easy, I found the SpiderVPN Pro for £19 GBP, this has dual band 5ghz wifi, dual band 2.4ghz wifi, 2 antennas for each, micro sd slot, 5 x gigabit ethernet ports, usb slot, micro sd slot, mini pcie slot, 512mb ram, 16mb rom.
You can also upgrade this from the web interface to Openwrt 18.06.2.
These are pretty cheap devices for what you get, but run very smoothly. I will try and update the OpenWRT wiki so that these models are included to save anyone else trying this, I'm also curious that there is no mention of either of these SpiderVPN devices on the OpenWRT forum.