The fun thing I find about these shapes is, you hardly need math, imagination is usually enough.
(hmm Imathination anyone?)
While I like to challenge myself and do them mathematically, the formula used in the end basically comes down to:
A figure eight going round a circle and doing a 180
@henryseg, thanks, very insightful, there goes my lazy sunday afternoon
Woah! The actual double Klein bottle in that paper DOES rock my boat