very nice fixes..
1. The filter on the bottom.. id keep it standard size..
goto a hardware store and check out hvac air filters u use for home.
See if there is a size u can work with. im thinking possibly u could fit a 12x24 filter easily..
http://www.airfiltersdelivered.com/...-filter.html?gclid=CJqwy7_EsroCFel_Qgod1ncAUQ
These filters should last 3-4 months each without replacement... a pack of 4 would last you one year.... each filter should cost arround 6-9 dollars unless u want a high merv index one which can filter out allergy pollen even. The thing is goto a local hardware store and see what filters they have which are cheap, that u can work with.
2. The PSU goes internally... i would try to honestly mount it under the motherboard... and route your cables on that bottom row...
Only your cables will be coming outside. Hold your original design layout for everything else....
U dont want the PSU outside as it will look messy... unless u can somehow incorporate the desk with it.
3. The average radiator is 5-6 inches wide... 1-2inches thick.... and 16 inches long....
Those sides are perfect to house a radiator standing.
Now render 2 rads standing on those wall... thats where ur cooling will be located.
were gonna aim for something simular to that... radiator, then a reservior next to it.. in pairs on each of your 9x22 walls...
The reserviors have a 60mm diameter... ~2.5inch diameter in customary system.
Since its in pairs, it will be very visually pleasing... symmetrical builds tend to do that cuz symmetry tends to show balance... and well, we all love balance.
Now we think of loop planning....
(dude i wish i had your wood skills...)