The Mirror-slicer Array for Astronomical Transients (MAAT) is a new IFU for the OSIRIS spectrograph on the 10.4-m Gran Telescopio CANARIAS (GTC) at La Palma, spectrograph that has been recently upgraded with a new detector and moved to the Cassegrain focus. Funding has been secured to build MAAT. We present the nearly final design, its expected performances, the different options that were studied, and an analysis of the spectrograph aberrations. MAAT will take advantage of the OSIRIS mask cartridge for multi-object spectroscopy. The IFU will be in a box that will take the place of a few masks. It is based on the Advanced Image Slicer (AIS) concept as are MUSE and KMOS on the VLT (among many others). The field is 10 '' x 7 '' with 23 slices 0.305 '' wide giving a spaxel size of 0.254 '' x 0.305 ''. The wavelength range is 360 nm to 1000 nm. The small space envelope, the maximum weight of the mask holder, and the curvature and tilt of the slit created additional design challenges. The spectral resolution will be about 1.6 times larger than with a standard slit of 0.6 '' because of the smaller size of the slices. All the eleven VPHs and grisms will be available to provide a broad spectral coverage with low to intermediate resolution (R=600 to 4100). To maximize the resolution of a spectrograph designed for a slit twice the width of the slices, we are in the process of measuring the wavefront of the spectrograph aberrations by using 2 out-of-focus masks with pinholes along the slit. We will then correct some of these aberrations with MAAT.