How to achieve a block of Z^(-0.5) in Simulink?
while @Andy aka captured the wider intent a practical implementation w.r.t. Simulink requires a re-evaluation of your sample time.

Remember in practice you can't have 1/2 a delay you can only have a different strobe. 

A simulink solution would require the discrete sampling time twice what you require so a \$Z^{-1}\$ block would behave like a 1/2 sample. Make use of specific sampling time on a per block or subsystem/enable basis