You don't have to buy an expensive tank unless you lack one, and would like to count with a bow shaped tank up front, if you have a traditional tank with middle handle, you have 2 low cost options:
1-Glue a medium d'ring on bow front, the same height as tank handle, secure with a strap, if tank swings laterally, place life vest or towels in each side so to center tank. 2-Using the 2 lateral d'rings most sibs have, place rear tank against bow, secure tight with roped handle to d'rings, as seen in pictures. Can place gear at both sides to fill available space.
Is a good issue to have tank secured on bow, when completely filled up, will count with 25 kilo weight to balance better sib's bow specially when no other mate is available, can place bow bag on tank top to add extra weight on bow. Pass engine hose by sib's middle or sideways, will depend on hose lenght. Preffer the second option, correctly tied doesn't move an inch.
The bow tank fit will depend entirely on sib's bow shape, square and round bows secures much better than sharp point bows like old Zodiac & Avon type bows.