No lenses at all.. Most leds have a 120° angle if not more.. Depending on the hight above the tank, in most cases not needed.
Types of leds?"Depends on you skillset.
The best leds out there at the moment are the COB led units. All up to date aquarium light or horticulture led light manufacturers use them e. g. Kessils uses COB leds.. Those popular flood lights used also use COB units..
Still these leds come in al kinds of shapes and powers. So it depends on what you are planning to grow and size of tank.. Medium sized tank with a height up to 40cm and medium light the easiest would be the COB units running on regular DC 12 volt e.g. widely used in the car or mobile home industry which are usualy dimmable via simple or programmable PWM controllers.
Do you go for other types e.g. 30 volt constant current, you need an accordingly powersupply and a driver providing the constant current.. Depending on the driver still PWM can be used as dim feature. But then again it requires some knowledge about that. Recently i found a nice
universal LED driver from Electro dragon.
These are somewhat configurable to drive leds from 6 to 30 volts and with changing the sense resistors it can be set to different currents and use an on board PWM tunable chip.. I'm going to use them to control my COB units.. But yet not found the time to build them.
Easy to say it aint so difficult.. But this is up to you.. I can't know, but options are numerous..
Constructionaly with acrylic, you should take the leds working temperatur, construction weight and strain into account.. Acrylic becomes rather soft at relative low temps if it warms up for long periods of time e.g. 12 hours a day. It could be OK for several months, but than starts to bend and this already at 40 to 50°C tops if the weight is out of balance.
COB leds can get pretty hot, again this depends on the power applied. SO they need a heatsink anyway or else burn out rather quickly.