I was in a similar situation and use them with good success for a season while figuring out how to get a proper cart. Just be sure to double check the dimensions so it fits. If it is a larger synth you can decide if some overhang is ok or not. I just strapped mine using smaller ratchet straps. If you have the budget you can buy better casters also. This does get pretty top heavy and can easily tip over!
CART - https://www.bandshoppe.com/shop/field-band-room/equipment-carts/rock-n-roller-r2g-micro-glider-multi-cart-66649/
SHELF FOR CART - https://www.bandshoppe.com/shop/field-band-room/equipment-carts/rock-n-roller-long-quick-set-shelf-for-r2-cart-663/
There is a bottom shelf attachment that I would recommend to store cables, monitors, etc. Helps with stability to have some weight on the bottom.
They offer some other configs but that will get you going. Take a look and see if that can help you. I imagine since they are available in so many places you should be able to get them rather quickly.
ANOTHER OPTION - Wood is expensive right now but you might have a parent or maintenance department that can slap something together for you pretty quick out of wood or angle iron.