How to make certain shapes to assemble together

I figure i need 2 small triangles, 2 medium sized triangles, 2 medium larger sized triangles and also a say a 10"x7" triangle! and last but not least 2 medium to large squares on each end to hold this mountain of cake together -Im hoping to make a 23" long and 8" wide, monstrosity. and yes It will resemble a mountain.

Never done this before -and I look foward to gobs and gobs of frosting to fill in my cracks. I realize I will have to deal with cake pans or such of prefurd sizes . I have made a many quick cake mixes etc. But nothing in this scale.
Could you please point me in the right direction as to where I go to purchase cake pan items and etc. (electric stove is tricky But have handle)- is there freezing involved?

anywho -thanks for any input and goodnight. TOM