As a brief update, I ended up getting a 4 position filter slider from AliExpress for about $75 (from memory). It's got 4 x 2" holes, with 4 x 1.25" adapter inserts. In my 8" collapsible dob I need to lower the tube about 20mm to reach focus, but it works like a dream in the 16" dob with no adjustment. Very smooth movement, and much easier to remove the filters compared with an enclosed filter wheel. As the filters are exposed you need to be a little careful waving your arms about, but generally it's made filter comparisons and viewing a lot more user friendly.
https://ae01.alicdn.com/kf/UT8o5aKX5...7b90eff20bac30