Product price, with width and length variants

-- HikaShop version -- :2.3.1
-- Joomla version -- : 3.3.0
-- PHP version -- : 5.3.28

I'm working on a new store where products are price according to the length and width.
One option would be to use variants, but the width and length combinations are many and the list is very long
I think the best solution would be two fields single dropdown, width and length, and once selected that will select a price. In this case, I tested width 2 custom fields table item single dropdown frontend ok, but i don’t what it is the next step to modify product price. I am installed HikaShopCustom Quantity plugin https://hikashop.com/forum/content-creation/866537-diff-prices-of-several-height-width-combinations.html but when I select dropdown value, price not working.
do not know if this is the best solution or should try other options

Attached a image with a sample table with width, length and price
Thank you
Javier

Hi Nicolas, thank you for your answer

i think that your solution , it would be the best solution
but, I have a problem when i work with 2 dropdown variations and one of the variation don't exist

for example
Variation Dropdown a: 70, 80
Variation Dropdown b: 5, 6 , 7

6 Combinations
70 x 5, x6, x7
80 x 5, x6, x7

and what happen when for example a product with the combination 70 x 7 don't exist ?
dropdown continue working, so user can select this combination,
even if the combination is not published
I have had read in other post that relation, refresh, with 2 dropdows don't exist by excess load on the code.

thank you for your answers
Javier