A while ago I experimented with custom supports that I modeled myself for this model: https://www.thingiverse.com/thing:1870525. I didn't try printing it until now. The first version is not maybe the most optimal, but I wanted to see how it would work. In this version the model is rotate 45 degrees on X axis, which makes the areas requiring supports quite small as they are just the corners of the rectangles. But this probably affects the overall print quality. But I got it printed yesterday after few retries due to too flimsy supports and curling caused by inadequate cooling.
The MMU seems to be working fine indeed, although the PVA does have the odd change of stringing during filament change, which usually causes some problems so it is a good idea to monitor the print.

I also had to implement support for no sparse layers in filaswitch because PVA on top of sparse infill layer was a total mess. So basically now it doesn't print the sparse infill layers which means that the tower height won't be the same as the model (unless the model has a tool change for every layer). This in turn means that the tower position needs to be far enough from the model to make sure that there won't be a collision with the model when the print head is lowered to the tower. It also saves some filament, making it almost like a purge bucket when only the tower brim and raft is 'wasted' compared to bucket.

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