You might try contacting Aristo; if the tube you are using is made by them for photographic use it certainly should cover 8X10 very nicely. If it is made for some other purpose you are in uncharted water! Given it is the correct tube, all those I am familiar with wind back and forth to make a pattern to cover the required area, then perhaps it is not working correctly somehow - blackened portions internally? That size MUST cover 8X10 reasonably well if it is working properly, pretty much regardless of what you have it sitting on (an old camera or whatever would not effect the light striking the negative. Something very strange here!